Composite or laminate articles formed of materials such as fibrous thermoset or thermoplastic materials are important to many industries, particularly the aircraft, transportation, and sporting industries, due to their high strength-to-weight and high stiffness-to-weight ratios. These articles can be formed of preforms that are shaped against a forming tool, and then solidified. Preforms typically are made up of individual layers of prepreg, that is, thermoset or thermoplastic resin containing embedded, aligned, and/or interlaced (e.g., woven or braided) fibers such as carbon fibers. The number of layers in a preform (and the preform's thickness) often is dictated by the strength and stiffness required of the resultant article. The layers typically are stacked such that the fibers of the various layers are oriented in a variety of directions, resulting in the desired stiffness of the shaped article in several directions, e.g., [0.degree./90.degree./.+-.45.degree.].sub.s.
Formation of a preform on a tool forming surface can be problematic, however, due to compression in portions of the preform which can lead to buckling and wrinkling of the preform. This problem is especially likely when a preform is formed against a tool surface having double curvature. The difficulty of formation increases as the thickness of the preform increases, thus formation of doubly curved articles made up of many layers can be difficult.
If a planar multi-layer preform is to conform to a nonplanar surface, deformation modes such as, inter-layer slippage (or shear), intra-layer slippage and transverse shear must occur to accommodate the complex shape and to alleviate the forces induced during formation. That is, layers of the preform must be made to slide relative to each other, and fibers within a layer must be made to slide (both longitudinally and transversely) relative to each other. If some of these or other in-plane deformation modes cannot be achieved, then the preform will possibly undergo buckling. If these modes can be achieved, then generally an article of double curvature can be formed that does not wrinkle or buckle, especially out of the plane of the article.
To form laminate articles of double curvature that are free of wrinkles, a hand lay-up process can be employed in which individual layers of prepreg material are placed manually ply by ply over a tool including a forming surface having a desired shape. This process, however, is very labor intensive.
An alternative method for forming such articles involves double diaphragm forming. This involves sandwiching a flat preform, defined by several layers of prepreg, between two diaphragms. The space between the diaphragms is evacuated, pressure is applied to one side of the arrangement (and/or vacuum to the other side) while the arrangement is adjacent a tool having a forming surface to which the preform is desirably conformed. The arrangement is set at a temperature at which the preform can achieve the shear modes described above, and the preform is deformed into the shape of the forming surface. The formed article then is removed from between the diaphragms.
Prior art methods of double diaphragm forming, however, suffer significant drawbacks. For example, after forming, one of the diaphragms (the "internal" diaphragm) is trapped between the part and the tool on which the part is formed. This can be problematic. During the forming of thermoset composites, elastomeric diaphragms that typically are used in the process are generally both relatively thick and compliant. Hence to insure high dimensional fidelity, it may be necessary to add an additional step to the process, whereby the trapped diaphragm is removed and the part is placed on a curing tool. In addition, the presence of the "internal" diaphragm for the double diaphragm processes prohibits the use of diaphragm forming as a sequential processes, whereby several layers are formed successively building up to a total desired thickness. The advantage of the sequential process is that it significantly increases the range of part size and degree of double curvature which can be formed.